This invention relates to a method for the multi-dimensional measurement of magnetic resonance in defined small volume regions of a solid-state sample in which the sample is placed in a uniform homogeneous magnetic field and, in a predetermined manner, irradiated with a series of high frequency pulses as well as subjected to a series of gradient magnetic field pulses in such a way that the spin magnetization to be measured is maintained for a period of time which is longer than a switching-off time of the gradient magnetic field pulses.
A method of this kind is disclosed in the Journal of Magnetic Resonance 66 (1986), pp. 530-535.
It is generally known that the technique of magnetic resonance, in particular nuclear magnetic resonance, is applicable to two- or three-dimensional measurements in defined small volume regions of samples. In particular, in medical research and in medical diagnostics, measurements of living or non-living human tissue are approached in this manner. In so doing, one distinguishes localized spectroscopic measurements in which a nuclear resonance spectrum of a small volume region only is recorded from imaging procedures with which a two- or three-dimensional representation of a body portion in the form of a picture of the spin density or relaxation time is generated and recorded.
Up to now, such volume selective measurements were largely undertaken of liquid samples. This is possible in a straight-forward manner since, due to molecular motion, liquid samples exhibit relatively narrow lines or slow pulses nuclear resonance signal decay times. During the relatively long time period during which this signal lasts, in the order of magnitude of many hundreds of milliseconds, the necessary measurement procedures can be undertaken in order to select and read out a small volume region. The measurement procedures consist, for the most part, of the application of gradient magnetic field pulses for which, for technical reasons, specific minimum switch-on and switch-off times are required.
If one carries out this type of measurement on solid-state bodies, the following problem results: Due to the internal field associated with solid-state bodies, the line width is appreciably larger and/or the decay time of the excited nuclear resonance signal is appreciably shorter, specifically of the order of magnitude of only several tens of microseconds. As a result, it is not technically possible to switch on and off the necessary read or phase encoding gradients during the extremely short time period over which this signal lasts, in particular, when strong gradients are required.
For this reason, in undertaking volume selective measurements on solid-state bodies in the few experiments known to date, one had utilized various special techniques to minimize the line width of the solid-state signal and/or extend the decay time of said signal.
In one known method of the kind mentioned at the outset, a multi-pulse sequence (MREV-8) is introduced in order to achieve a narrowing of the lines. This prior art multi-pulse sequence is, furthermore, developed in such a way that a "storage" of the magnetization is established for a period of time which is sufficient to allow the gradient magnetic field pulses to once again being switched-off.
The prior art method has, however, the disadvantage that read gradients must be utilized in order to record the signal. These read gradients produce a degradation in homogeneity of the constant magnetic field and, therewith, a systematic line-broadening. The prior art method is, therefore, limited in applications involving samples with narrow lines, and does not facilitate line-shape measurements, since, for the reasons mentioned, the measured line-shape is widened as compared to the pure line-shape as a result of the read gradients.
It is also known that, in solid-state measurements, problems associated with wide lines and/or fast signal decays are mitigated against by rotating the sample around the so-called "magic angle". In order to do so, the rotation must be undertaken using a relatively high rotation frequency of many kHz. The fast rotation of the solid-state body simulates the fast molecular motion of liquids, thereby averaging out the local fields. However, at such high rotational frequencies, the samples must be, for mechanical reasons, rotationally symmetric. Even with rotationally symmetric samples, there is nevertheless the danger that a deformation of the sample will occur due to the very high rotational frequencies.
This prior art method is, therefore, not applicable to measurements of biological samples such as extracted teeth. Such natural samples have random irregular shapes and sample deformation is not acceptable in cases where the biological sample is still needed after the measurement. This is, for example, the case in modern dentistry when an unhealthy tooth is first extracted, measured, then treated and, finally, reimplanted.